This document relates to optical resonators, including optical resonators that support optical whispering gallery modes, and devices based on optical resonators.
Optical resonators may be used to spatially confine resonant optical energy in a limited cavity with a low optical loss. The resonance of an optical resonator can provide various useful functions such as optical frequency references, optical filtering, optical modulation, optical amplification, optical delay, and others. Light can be coupled into or out of optical resonators via various coupling mechanisms according to the configurations of the resonators. For example, Fabry-Perot optical resonators with two reflectors at two terminals may use partial optical transmission of at least one reflector to receive or export light.
Optical whispering gallery mode (WGM) resonators confine light in a whispering gallery mode that is totally reflected within a closed circular optical path. Unlike Fabry-Perot resonators, light in WGM resonators cannot exit the resonators by optical transmission. Light in a WGM resonator “leaks” out of the exterior surface of the closed circular optical path of a WGM resonator via the evanescence field of the WGM mode. An optical coupler can be used to couple light into or out of the WGM resonator via this evanescent field
Optical resonators can be used to generate resonator resonances in frequency as frequency references for a wide range of applications. For example, the resonance of an optical resonator can be used as a reference to which a frequency of laser can be locked to achieve a stabilized laser operation.
One technical challenge associated with using optical resonators as frequency references is stabilization of a resonance of an optical resonator against drifts and fluctuations of the resonance caused by various factors because the resonator is subject to internal changes and external perturbations. For compact optical resonators, including optical whispering gallery mode resonators with a dimension on the order of millimeters or less (e.g., 10˜102 microns), it is difficult to stabilize the optical resonators and their resonances.